1. Field of the Invention
This invention relates to hydraulic hybrid motor vehicles and to vehicles with hydrostatic drive systems.
2. Description of the Related Art
Series hydraulic hybrid powertrains are being studied as a way to improve the fuel efficiency of motor vehicles. Commonly-assigned U.S. Pat. No. 6,719,080 to Gray, which is incorporated herein by reference, describes one example of a hydraulic hybrid powertrain motor vehicle utilizing a series mode of operation. In one aspect of the Gray '080 patent, an internal combustion engine drives a hydraulic pump to pressurize working fluid from a low pressure (e.g., a few hundred psi) to a high pressure (e.g., several thousand psi). The high pressure fluid may then be (1) utilized to drive a hydraulic motor to propel the vehicle, and/or (2) used to help fill a high pressure accumulator for energy storage and later usage (e.g., for further/later vehicle propulsion). Some hydraulic hybrid powertrains that incorporate a mechanical drivetrain may still use this type of series hydraulic operation for certain vehicle modes.
The Gray '080 patent partially addresses a condition where there is a higher vehicle torque request than can be supplied by full displacement of the hydraulic drive motor(s) at the instant hydraulic system pressure. For this condition, the Gray '080 patent discloses valving out (i.e., closing a valve to shut off flow to) the high pressure accumulator to temporarily remove the high pressure accumulator from the hydraulic circuit, thus temporarily operating in a direct “hydrostatic” mode (defined here as a “constant volume” mode), which allows the engine-driven pump to quickly raise the system working pressure to make it possible to satisfy the vehicle's requested torque. See U.S. Pat. No. 6,719,080 at col. 12, lines 28-57. However, as indicated in Gray, this also creates a risk of exceeding the maximum rated pressure for the system. Gray therefore teaches managing of the engine power to avoid exceeding the rated system pressure. See U.S. Pat. No. 6,719,080 at col. 12, lines 57-61. As a practical matter, without further protection, this approach would mean that the system could not be operated at or very near the maximum pressure limit for the system without jeopardizing safety and reliability of the system, and would instead require operating at pressures comfortably lower to prevent safety concerns, which would therefore reduce fuel efficiency and vehicle performance. Additional or alternative safety measures are therefore desirable.
U.S. Pat. No. 6,427,441 to Wustefeld discloses a hydrostatic vehicle drive with a pressure-reducing valve to avoid exceeding maximum rated hydraulic system pressures. However, Wustefeld does not preserve the energy lost in reducing the instant hydraulic system pressure, which would thus result in a loss of fuel efficiency if applied in a motor vehicle. A more efficient approach is therefore also desirable.
Therefore, there is a need for a device or method to provide safe and efficient hydrostatic operation, particularly for series mode of operation on any hydraulic hybrid motor vehicles.